The "d3dx" prefix stands for , and the number "3_30" refers to a specific version of the DirectX SDK (Software Development Kit). Specifically, d3dx3_30.dll is part of the DirectX 7.0a runtime, released around 1999. It was crucial for rendering 2D and 3D graphics in older titles like Unreal Tournament , Deus Ex , The Sims (original), and Age of Empires II .
: If the web installer doesn't work, you can use the DirectX Redist (June 2010) package, which contains all legacy files and overwrites existing ones. d3dx3 30.dll.